1. Field of the Invention
The present invention generally relates to an electromechanically controlled device and more particularly, although not specifically so limited, to an automatic camera diaphragm aperture control device of the type which, while stopping-down the diaphragm of an objective lens, measures the intensity of the light coming from an object to be photographed and having passed through the aperture, and stops the stop-down operation when the light measurement reaches a predetermined relationship with a predetermined value, thereby determining a diaphragm aperture.
2. Description of the Prior Art
There have been marketed cameras equipped with the aforementioned type of diaphragm aperture control device, which employs an electromagnetic or electromechanical mechanism to convert an electric signal into a mechanical motion. For aperture control, the diaphragm of the objective lens is rapidly stopped down from the full aperture towards the minimum aperture, and an electric signal is generated when the aperture reaches a desired value during the stopping-down of the diaphragm. In response to this electric signal, the electromagnetic mechanism is actuated to stop the operation of a diaphragm aperture stopping-down member or a member interlocked therewith thereby determining a diaphragm aperture. In this case, a slight time lag occurs between the generation of the electric signal and the time when the stopping-down member is actually restrained to stop its movement. This requires the aperture stop signal to be generated at a time equal to the time lag before the aperture reaches a desired value so that the aperture may be controlled to the desired value.
However, since the aperture stop-down operation accelerates, the time lag to be estimated should be varied in accordance with the rate of the stopping-down operation to determine the aperture more accurately. Japanese Laid-Open Patent Application No. Tokkai-Sho 56-4125 discloses a diaphragm aperture control device which, to meet such requirements, varies the time interval to be estimated for the generation of an aperture stop signal before the actual arrestment of the diaphragm driving member, on the assumption that the rate of aperture stopping-down is a function of the elapsed time from the start of the stop-down operation. The disclosed device has a circuit constructed such that a light measurement voltage representative of a suitable shutter speed for the measured object brightness, and a voltage signal representative of a preselected shutter speed are applied to a voltage comparator, the output of which is inverted when these two input voltages reach a predetermined relationship, thereby actuating an electromagnetic mechanism. In this circuit, a voltage signal with a value corresponding to each elapsed time is generated at a plurality of predetermined elapsed time points from the aperture stop-down operation. The voltage of the last signal in each time interval between the elapsed time points is added, for example, to the voltage signal representative of the predetermined shutter speed, so that the output of the voltage comparator may be inverted earlier by the time corresponding to the added voltage.
As is generally known, a wide variety of exchangeable lenses are available to a single-lens reflex camera. The device disclosed in the No. Tokkai-Sho 56-4125, if made effective for such exchangeable lenses, requires the aperture stop-down characteristics of each of the exchangeable lenses to be the same. In an actual exchangeable lens system, however, the amount of stopping-down from the full aperture with the displacement of a lens aperture stop-down lever which is driven by the diaphragm drive member of a camera body, often varies with the type of lens. Consequently, the device is not applicable to the camera including such exchangeable lens systems.